Generally, a high temperature superconducting film has been developed in the form of a film in which a superconducting layer is coated on an oxide substrate or a metal substrate. A superconducting wire rod fabricated by coating one or more oxide film layers on a metal substrate and depositing a superconducting layer thereon is referred to as “a superconducting thin film wire rod”. The superconducting thin film wire rod has a structure formed of four layers, that is, a metal substrate layer, an oxide buffer layer, a superconducting layer and a stabilizing layer. Techniques for forming a superconducting layer include a Pulsed Laser Depositon (PLD) method, a Metal Organic Deposition (MOD) method, a Metal Organic Chemical Vapor Deposition (MOCVD) method, a Co-Evaporation method and the like.
The most important requirement for the superconducting layer is that the critical current density must be high. In particular, the critical current density must be as high as possible even when a large magnetic field is applied in an arbitrary direction. The limit of the critical current density is determined by the action of pinning centers that fix magnetic flux lines so that the magnetic flux lines are not moved by Lorentz' force when the magnetic flux lines, which intrude from the outside and are distributed in the superconducting layer, are moved by Lorentz' force.
Generally, the pinning centers are defects existing in a superconductor, and are classified into three types. That is, zero-dimensional pinning centers are point defects having almost the same diameters as the magnetic flux lines, one-dimensional pinning centers are dislocations and columnar defects, two-dimensional pinning centers are crystal grain boundaries and planar defects, and three-dimensional pinning centers are precipitates and second phase particles. These pinning centers have different anisotropies of critical current depending on the direction of the magnetic field due to the geometrical differences therebetween, and the characteristics are specified for the pinning centers.
Meanwhile, multi-directional magnetic fields are three-dimensionally formed in the interior, which is wound with a superconducting wire rod, even in the case where the superconducting wire rod is wound and then used as an electromagnet or is used for some other purpose. Accordingly, magnetic anisotropy with respect to critical current density of the superconducting wire rod is required to have various characteristics, therefore various geometrical conditions of the pinning centers are also necessary.
The planar defects are pinning centers inherently existing in a superconducting crystal structure, and copper oxide (CuO) face serves as an example thereof. Further, a crystal grain boundary is also an effective pinning center under a high magnetic field in the case of a low angle misorientation grain boundary. Columnar defects, which are generally generated at the time of growth of a superconducting film, and rod-shaped defects and dislocations, which grow using nanoparticles artificially added as seeds, are well known as the one-dimensional linear defects.
In the MOD method, the artificially added nanoparticles form point defects. It has been reported that the linear defects are formed by adding nano-oxide particles when a high temperature superconducting film is formed on a substrate using the PLD method. That is, a method of increasing the pinning force of the magnetic flux lines using a technique of forming nanorod-shaped linear defects by non-uniformly growing BaZrO3 due to lattice mismatch of superconducting phases and BaZrO3 and nanoparticles and BaZrO3 when Yttrium Barium Copper Oxide (YBCO) target mixed with BaZrO3 nano-oxide particles is deposited using the PLD method (J. L. Macmanus-Driscoll, Nature Materials 3, 2004, p. 439; M. Mukaida, JJAP, 43, 2004, p. 1623; A. Goyal et al, Supercond. Sco. Technol. 18, 2005, p. 1533), and a method of increasing the pinning force of the magnetic flux lines using a technique of forming dislocations, which are linear defects, on Y2O3 seeds by previously dispersing and forming the Y2O3 on a substrate in the process of deposition using the PLD method and then depositing the YBCO superconducting film using the PLD method (K. Matsumoto, JJAP 43, 2004, p. 1623), are representative methods that have been reported.
However, the conventional techniques have problems in that, when nanoparticle material is mixed with a superconductor and is then deposited, the nanoparticle material becomes nanoparticles while forming clusters, wherein the clusters are formed depending on thin film growth conditions, so that the conventional techniques are not desirable methods of forming the pinning centers.